Solution-Processed Nb:SnO2 Electron Transport Layer for Efficient

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Solution-Processed Nb:SnO2 Electron Transport Layer for Efficient Planar Perovskite Solar Cells Xiaodong Ren,† Dong Yang,*,† Zhou Yang,† Jiangshan Feng,† Xuejie Zhu,† Jinzhi Niu,† Yucheng Liu,† Wangen Zhao,† and Shengzhong Frank Liu*,†,‡ †

Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology; School of Materials Science & Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China ‡ Dalian Institute of Chemical Physics, Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences, Dalian 116023, P. R. China S Supporting Information *

ABSTRACT: Electron transport layer (ETL), facilitating charge carrier separation and electron extraction, is a key component in planar perovskite solar cells (PSCs). We developed an effective ETL using low-temperature solution-processed Nb-doped SnO2 (Nb:SnO2). Compared to the pristine SnO2, the power conversion efficiency of PSCs based on Nb:SnO2 ETL is raised to 17.57% from 15.13%. The splendid performance is attributed to the excellent optical and electronic properties of the Nb:SnO2 material, such as smooth surface, high electron mobility, appropriate electrical conductivity, therefore making a better growth platform for a high quality perovskite absorber layer. Experimental analyses reveal that the Nb:SnO2 ETL significantly enhances the electron extraction and effectively suppresses charge recombination, leading to improved solar cell performance. KEYWORDS: perovskite, Nb:SnO2, low temperature, solution processing, solar cells

1. INTRODUCTION A new class of organic−inorganic halide perovskite materials have been developed for high-performance photovoltaic and optoelectronic applications, such as solar cell, light-emission diode, laser, photodetector, etc., thanks to their remarkable photophysical properties including appropriate bandgap, high optical absorption coefficient, high charge carrier mobility,1−6 low trap density, and surprisingly high tolerance to defects.7−11 In only a few years, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been rapidly increased from 3.8% to >22%.12−23 Theoretical calculation reveals that the PCE of a single-junction PSC may be increased up to 33.5%,24 which is comparable with the most popular inorganic solar cells including single-crystalline silicon, CdTe, and copper indium gallium selenide (CIGS). Moreover, it can be fabricated using facile solution processing at low temperature, making it particularly attractive for low-cost comercialization.25 Most high PCE PSCs are based on meso-superstructure with a layer of mesoscopic metal oxide such as TiO2, Al3O2, ZrO2, etc. The challenge is that it often requires complicated and high-temperature (>450 °C) processing.15,26,27 In comparison, the planar-type PSC has witnessed an unprecedented development recently because of its simple structure, relatively lowtemperature fabrication, and facile preparation processes.7,28,29 In the planar-type device configuration, the perovskite absorber layer is usually sandwiched between a layer of electron © 2016 American Chemical Society

transport layer (ETL) and another hole transport layer (HTL).30 The interface layers aid effective charge extraction and inhibit interfacial carrier recombination for improved cell characteristics.31,32 There have been efforts studying ETL-free device configuration; however, their efficiency is lower compared with the devices with ETLs.31,33 For the regular PSCs architecture, the 2,2′,7,7′-tetrakis(N,Ndi-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD)2,8,34−37 and poly(triarlyamine) (PTAA)18,21,38 with appropriate dopants are most often used HTLs, while a few metal oxides including TiO239−46 and ZnO47,48 are most popular ETLs, in addition to some ternary compounds also used as ETLs, such as Zn2SnO4,49,50 SrTiO3,51 etc. Theoretically, an ideal ETL needs to provide: (i) good optical transparency, (ii) decent electrical conductivity, (iii) large electron mobility, (iv) low-temperature processability, and (v) well-matched energy levels with the perovskite material.52 Unfortunately, the best TiO2 and ZnO ETLs are often attained using high-temperature treatment.53−55 More recently, tin oxide (SnO2) is developed as an effective ETL due to its high optical transmission and low-temperature preparation.32,56,57 Snaith et al. formed a SnO2 ETL by chemical Received: October 20, 2016 Accepted: December 20, 2016 Published: December 20, 2016 2421

DOI: 10.1021/acsami.6b13362 ACS Appl. Mater. Interfaces 2017, 9, 2421−2429

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ACS Applied Materials & Interfaces

Figure 1. (a) The typical XPS survey of Nb:SnO2 film. High-resolution XPS (b) Sn 3d, (c) O 1s, and (d) Nb 3d peaks of the Nb:SnO2 film deposited on a glass substrate.

bath deposition at 80 °C to achieve high PCE up to 17.1%.58 Alex et al. employed nanocrystalline SnO2 as ETL to obtain PCE as high as 18.8%.25 Miyasaka and Kuang et al. developed SnO2 films at low temperature and attained respectable efficiencies over 13%.59,60 Grätzel et al. achieved 18% PCE using atomic layer deposition.61 Lately, Yan et al. raised the PCE to 19.12% by using SnO2 cooperate with fullerene.62 Herein, we report our development that, by doping SnO2, we realized a more effective low-temperature solution-processed (LTSP) Nb-doped SnO2 (Nb:SnO2) ETL. Comparing to the pristine SnO2, the PCE of Nb:SnO2 based PSC is improved to 17.57% from 15.13%. The PCE improvement is attributed to improved properties including higher electron mobility, good electrical conductivity, and faster electron extraction.

solution. A SnO2 ETL with different thickness was prepared by spincoating different concentration of SnCl2·2H2O at 3000 rpm in ambient condition.56 The layers were then sintered in air at 190 °C for 60 min. The Nb:SnO2 mixed precursor solution was prepared by adding different niobium ethoxide into the SnCl2·2H2O solution. Nb:SnO2 ETLs were fabricated by spin-coating at 3000 rpm in ambient condition, and the films were annealed at 190 °C in air. Once they cooled to room temperature, the samples were treated again with oxygen plasma for 15 min before the perovskite deposition. FAI (172 mg, 1.0 M), 507 mg of PbI2 (1.1 M), 22.4 mg of MABr (0.2 M), and 73.4 mg of PbBr2 (0.2 M) were dissolved in 1 mL of anhydrous DMF/DMSO = 4:1 (v/v) to prepare the (FAPbI3)0.85(MAPbBr3)0.15 perovskite precursor solution.61 The solution was stirred at room temperature for 12 h. The solution was then spincoated onto the FTO/ETL substrate by a consecutive two-step process at 1000 and 4000 rpm for 20 and 40 s, respectively. During the second step, 200 μL of chlorobenzene was added onto the substrate before the spinning ended. The substrates were then heated at 150 °C for 10 min in a nitrogen-filled glovebox; the film color changes from orange-red to dark brown. The 90 mg/mL spiro-OMeTAD in 1 mL of chlorobenzene with addition of 36 μL of t-BP and 22 μL of Li-TFSI solution (520 mg in 1 mL acetonitrile) was spin-coated on the perovskite films at 5000 rpm for 40 s. The samples were kept in a desiccator for overnight. Finally, 80 nm gold electrodes were deposited using a thermal evaporator. 2.3. Characterization. The X-ray diffraction (XRD) was performed on a Bruker D8 (Cu Kα radiation, λ = 1.5418 Å). The absorption and transmittance spectra were measured using a UV−vis− near-IR spectrometer (PerkinElmer, Lambda 950). The X-ray photoelectron spectroscopy (XPS) was performed on a photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific). Field emission scanning electron microscopy (SEM, HITACHI, SU-8020) was utilized to investigate the surface morphologies for pristine SnO2, Nb:SnO2, and perovskite films. Photoluminescence (PL; excitation at 532 nm) and time-resolved PL (TRPL; excitation at 405 nm and emission at 760 nm) were measured with Edinburgh Instruments Ltd. (FLS980). The J−V curves were measured using a computer-

2. EXPERIMENTAL SECTION 2.1. Materials. 4-tert-Butylpyridine (t-BP), and Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI) were purchased from Aladdin reagent. PbI2 (99.99%), PbBr (99.99%), niobium ethoxide (99.999%), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were ordered from Alfa-Aesar. SnCl2·2H2O (99.99%) was purchased from Aladdin. Ethanol, diethyl ether, acetonitrile, hydrobromic acid (HBr, 57 wt % in water, 99.99%) and hydroiodic acid (HI, 57 wt % in water, 99.99%), chlorobenzene (≥99.0%), formamidine (FA), and methylamine (MA) (33 wt % in absolute ethanol) were purchased from Sinopharm Chemical Reagent Corporation Co., Ltd. Spiro-OMeTAD (≥99.0%) was ordered from Dalian Lichuang Technology Co., Ltd. MABr and FAI were synthesized as reported in our earlier work to achieve record efficiencies.7,9,34 All solvents were used without any further purification. 2.2. Device Fabrication. The fluorine-doped tin oxide (FTO) glass was cleaned using acetone, isopropanol, and ethanol separately in ultrasonic bath for 30 min each, then dried by flowing nitrogen. The FTO substrates underwent an oxygen plasma treatment for 15 min before they were used for spin-coating SnCl2·2H2O in ethanol 2422


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Figure 2. (a) XRD patterns of pristine SnO2 and Nb:SnO2 films with different Nb contents deposited on bare glass substrates. (b) Details of the XRD patterns at (111) peak.

Figure 3. Top-view SEM images of (a) SnO2 and (b) Nb:SnO2 films coated on FTO substrates. AFM height images of the (c) SnO2 and (d) Nb:SnO2 films. controlled Keithley 2400 source under ambient condition, and the illumination intensity was adjusted at 100 mW cm−2 (AM 1.5G, SANEI ELECTRIC XES-40S2-CE solar simulator), as calibrated by a NREL-traceable KG5 filtered silicon reference cell. The device area of 9 mm2 was defined by a metal mask. All devices scanned with reverse and forward under standard test procedure at a scan rate of 0.24 V s−1. The external quantum efficiency (EQE) was characterized on the QTest Station 500TI system (Crowntech Inc., USA). The monochromatic light intensity for EQE was calibrated using a reference silicon photodiode.

reveals that there are two different peaks located in 486.78 and 495.23 eV, corresponding to Sn 3d5/2 and Sn 3d3/2, respectively, giving a spin−orbit coupling of 8.45 eV, the signature of Sn4+.63 In addition, the main binding energy of 530.45 eV is attributed to the O 1s, as shown in Figure 1c, indicating the O2− state in SnO2. The higher binding energy (531.77 eV) can be assigned to the chemisorbed oxygen atoms or hydroxyl groups.56 Figure 1d gives the Nb 3d5/2 and Nb 3d3/2 peaks at 206.87 and 209.64 eV, respectively.64 It is clear from above that Nb5+ doped SnO2 is successfully attained. The crystallinity and the preferred crystal orientation of pristine SnO2 and Nb:SnO2 films were analyzed using XRD, as shown in Figure 2a. The only one peak at 31.80° is attributed to the rutile-type tetragonal structure of SnO2 [JCPDS50− 1429], corresponding to the (111) diffraction. Obviously, the samples remain the rutile phase irrespective of the Nb doping, whereas the (111) rutile peak slightly shifts to a smaller θ angle with increased Nb doping (Figure 2b) as expected by the bigger ionic radius of Nb5+ (0.70 Å) compared to that of Sn4+ (0.69

3. RESULTS AND DISCUSSION 3.1. Characterization of the SnO2 and Nb:SnO2 Films. The SnCl2·2H2O in ethanol solution with different Nb contents was spin-coated onto the glass substrates, followed by thermal annealing at 190 °C for 60 min. The composition and bonding type of the Nb:SnO2 film were measured using XPS. The typical XPS spectra of Nb:SnO2 is shown in Figure 1a. Clearly, the O, Sn, and Nb peaks are centered at ∼530, ∼487, and ∼495 eV, respectively. High-resolution Sn 3d (Figure 1b) spectrum 2423


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Figure 4. (a) I−V characteristics of SnO2 and Nb:SnO2 films. (b) Transmission spectra of bare FTO, pristine SnO2, and Nb:SnO2 with different Nb contents.

Figure 5. (a) Device structure of the PSC using LTSP Nb:SnO2 as ETL. (b) Cross-sectional SEM image of the PSC completed device.

Å), suggesting effective Nb5+ doping into the SnO2 host lattice.58 Figure 3a,b displays top-view SEM images of the SnO2 and Nb:SnO2 films; both appeared to be flat, uniform, and pinholefree. The surface morphology of the Nb:SnO2 is similar to that of the pristine SnO2 film, showing that the Nb doping does not change the film morphology as expected. Figure 3c,d shows atomic force microscopy (AFM) height images of the SnO2 and the Nb:SnO2 films; the root-mean-square roughness is decreased from 5.73 to 5.08 nm by the Nb doping. Note that the smoother surface is beneficial to form a high-quality perovskite layer.65 As electrical conductivity is a critical figure of merit for the ETL, Figure 4a and Supporting Information Table S1 shows the electrical conductivity of ETLs deposited on FTO and bare glass substrates. It is apparent that the electrical conductivity of the SnO2 film is significantly increased by the Nb doping regardless of different substrates. Figure 4b gives optical transmission spectra of the SnO2 and Nb:SnO2, both showing excellent transmittance in the wavelength range of 400−800 nm. In addition, the electron mobility is measured using electron-only devices using space-charge-limited-current (SCLC). The details are shown in the Supporting Information. Supporting Information Figure S1 shows the current density− voltage (J−V) curves based on SnO2 and Nb:SnO2 films fitted using the Mott−Gurney law.66 The electron mobility is increased from 1.02 × 10−4 to 2.16 × 10−4 cm2 V−1 s−1 by the Nb doping. A low mobility of the electron transport layer indicates more traps existed leading to charge accumulation at the interface and inferior charge transport.67 The dark current−voltage (I−V) analysis for electron-only devices were used to quantify the electron trap-state density in the SnO2 and Nb:SnO2 films (Supporting Information Figure S2). Clearly, the linear relationship (blue line) indicates an ohmic response of the

electron-only device at low bias voltage. Then the current quickly increases nonlinearly (green line) when the bias voltage exceeds the kink point, demonstrating that the trap states are completely filled. The trap state densities are calculated by following eq 1:34

VTFL =

entL2 2εε0

(1)

where e is the elementary charge of the electron, L is the thickness of the SnO2 and Nb:SnO2 film, ε is the relative dielectric constant of the SnO2, ε0 is the vacuum permittivity, and nt is the trap-state density. The VTFL can be obtained from the kink point in the dark I−V curve. The VTFL of the devices based on SnO2 (1.88 V) is higher than that of Nb:SnO2 (1.37 V). The density of electron traps in the SnO2 is determined 2.39 × 1015 cm−3 higher than that of the Nb:SnO2 (1.74 × 1015 cm−3). Clearly, Nb doping effectively passivates electron traps, leading to higher electron mobility. 3.2. Photovoltaic Performance. The above measurements show that the SnO2 properties are significantly improved by the Nb doping including enhanced electron mobility, smoother surface, and larger electrical conductivity. The excellent performance inspired us to design and fabricate PSCs using the Nb:SnO2 ETL. Figure 5a shows the device structure, wherein the FTO is employed as the cathode, SnO2 or Nb:SnO2 film as the ETL, the (FAPbI3)0.85(MAPbBr3)0.15 as the absorber layer, the Spiro-OMeTAD as the HTL, and the gold layer as the anode. Figure 5b gives a cross-sectional SEM image of the completed device; the grain boundary is vertical to the substrate, benefiting to the carriers transport collection.36 Supporting Information Figure S3a shows the top-view SEM of the perovskite film deposited on the LTSP Nb:SnO2 ETL. The perovskite film exhibits smooth surfaces, big crystalline size and good coverage. The XRD pattern shows that the perovskite film 2424


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Figure 6. (a) The J−V curves of PSC devices based on pristine SnO2 and Nb:SnO2 under reverse and forward scanning directions. (b) EQE of the champion devices based on SnO2 and Nb:SnO2 ETLs.

Table 1. Key Parameters of Champion PSCs Based on Pristine SnO2 and Nb:SnO2 ETLs SnO2 Nb:SnO2

reverse forward reverse forward

Voc (V)

Jsc (mA cm−2)

FF

PCE (%)

Rs (Ω·cm2)

Rsh (kΩ·cm2)

1.06 0.98 1.08 1.04

21.65 21.57 22.36 21.51

0.659 0.526 0.727 0.570

15.13 11.12 17.57 12.75

8.58 12.97 7.09 10.84

2.95 0.54 5.84 1.49

Figure 7. Histograms of PCEs measured for 20 cells using pristine SnO2 (a) and Nb:SnO2 (b) ETLs.

the device performance at 0.50%. Figure 6a shows the J−V curves of the champion devices based on both SnO2 and Nb:SnO2 ETLs measured under reverse and forward scan directions. Table 1 lists the key J−V parameters of the champion devices using both ETLs. The device based on the pristine SnO2 ETL using reverse scan direction gives a PCE of 15.13% with Jsc 21.65 mA cm−2, Voc 1.06 V, and FF 0.659. While the Nb is doped into the SnO2 ETL, the PCE is rapidly increased to 17.57%. Compared to the control device, all key J−V parameters are significantly improved. The larger Jsc and FF may be ascribed to the high electron mobility and electrical conductivity of the Nb:SnO2 ETL. The high Voc is likely due to the reduced charge recombination and improved electron extraction. Figure 6b shows the EQE and integrated current based on various ETLs. The EQE integrated current density for the pristine SnO2-based cell is 21.11 mA cm−2, and it is increased to 21.79 mA cm−2 for the Nb:SnO2-based device, in good agreement with the J−V measurement value. The reflection and transmission of a complete device are characterized, and the internal quantum efficiency (IQE) is calculated from the EQE and reflection and transmission spectra, with the results shown in Supporting Information Figure S6. Clearly, the IQE of the Nb:SnO2-based device at

is similar compared to those reported in prior art (Supporting Information Figure S3b).62 It is found that the PSC performance can be significantly affected by the SnO2 ETL thickness. The device with too thick ETL leads to a high series resistance (Rs) and a small shunt resistance (Rsh), reducing the Jsc and fill factor (FF). However, when the ETL is thinner, pinholes are seen in the film, leading to direct contact between the perovskite and the FTO electrode and serious carrier recombination. In the present study, the SnO2 film thickness is adjusted by controlling SnCl2·2H2O− ethanol solution concentration. Supporting Information Figure S4 shows J−V curves as a function of SnO2 ETL thickness. The key J−V parameters are summarized in Supporting Information Table S2. The PCE is optimized to 15.13% when 60 nm thick SnO2 is used by controlling the precursor concentration at 0.07 mol mL−1. Supporting Information Figure S5 shows the device performance optimized as a function of Nb content, with key J−V parameters summarized in Supporting Information Table S3. It appears that the PCE is increased from 15.13% to 17.57% when the Nb content is changed from 0 to 0.50%. When the Nb content is further increased to 2.00%, the PCE drops to 15.11%. It is apparent that there is an optimum Nb content for 2425


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Figure 8. (a) Steady-state PL spectra of the FTO/perovskite, FTO/SnO2/perovskite, and FTO/Nb:SnO2/perovskite samples. (b) TRPL of perovskite absorbor deposited on different substrates.

∼480 nm is up to 99%, indicating that the carriers are effectively collected in the PSC using the Nb:SnO2 ETL. To study the reproducibility of the material and process, we prepared more than 20 individual devices each using both SnO2 and Nb:SnO2 ETLs. Figure 7 shows the PCE histogram, with the statistics summarized in Supporting Information Table S4. Clearly, all key J−V parameters displays narrower distribution with smaller standard deviation for the Nb:SnO2 based cells, indicating good reproducibility. In addition, the stability of the devices based on SnO2 and Nb:SnO2 ETLs were studied. Using the bare devices without any encapsulation, we measured J−V on a daily basis for 12 d. Between measurements, the cells were kept in lab under ambient condition. The PCE value remained at 92% and 90% of its initial efficiency after 12 d for the devices based on SnO2 and Nb:SnO2 ETL, respectively, as shown in Supporting Information Figure S7. We attribute the respectable stability to the high-quality perovskite film.2 In other words, Nb doping has no negative impact on device stability. 3.3. Recombination. To gain insight on the electron extraction and transport mechanism, the steady-state PL and TRPL were measured for the perovskite absorber layer deposited on both ETL-based substrates. Figure 8a shows the PL spectra of the FTO/perovskite, FTO/SnO2/perovskite, and FTO/Nb:SnO2/perovskite samples. The FTO/perovskite shows apparently the highest PL intensity, indicating the serious recombination in the sample. The FTO/Nb:SnO2/ perovskite sample shows the lowest, even lower than that of FTO/SnO2/perovskite, demonstrating more effective electron extraction. Figure 8b displays the TRPL spectra for the same samples. The PL decay time and amplitudes are fitted using exponential eq 2 f (t ) =

∑ Aiexp(−t /τi) + K i

Table 2. Parameters of the TRPL Spectroscopy Based on the FTO/Perovskite, FTO/SnO2/Perovskite, and FTO/ Nb:SnO2/Perovskite, Respectively sample

τave (ns)

τ1 (ns)

amplitude τ1 (%)

τ2 (ns)

amplitude τ2 (%)

932.65 142.14

12.61 10.90

60.71 64.41

940.63 147.50

39.29 35.59

42.71

6.18

64.10

45.49

35.90

FTO/perovskite FTO/SnO2/ perovskite FTO/Nb:SnO2/ perovskite

of the fast decay time increased to 35.90%. The average lifetime (τave) is estimated using eq 3. τave =

∑ Ai τi 2 ∑ Ai τi

(3)

Compared to the FTO/perovskite sample, the average decay time of FTO/SnO2/perovskite significantly is reduced to 142.14 from 932.65 ns. It is further dropped to 42.71 ns when the Nb:SnO2 ETL is used, demonstrating that the electron transfer is faster from the perovskite film into the Nb:SnO2 ETL, as witnessed by stronger steady-state PL quenching in the Nb:SnO2/perovskite sample (Figure 8a). The faster electron injection rate from the perovskite to the Nb:SnO2 ETL is beneficial to the charge separation and effectively suppressed charge recombination at the perovskite/ ETL interface, leading to higher Jsc and Voc values, in good agreement with the J−V and IPCE measurement.

4. CONCLUSION In conclusion, we have demonstrated that LTSP Nb:SnO2 is an excellent ETL for high-performance perovskite solar cells, with the champion cell showing significantly higher PCE of 17.57% than that of devices based on pristine SnO2 ETL. The outstanding performance using the Nb:SnO2 ETL originates from improved surface morphology, higher electron mobility, larger electrical conductivity, and enhanced electron extraction. Moreover, considering the low-temperature solution-processable fabrication, the Nb:SnO2 is demonstrated to be an effective ETL for low-cost PSCs.

(2)

where Ai is the decay amplitude, τi is the decay time, and K is a constant for the baseline offset. All parameters are listed in Table 2. When the perovskite is deposited directly on the FTO, its PL decay time are τ1 = 12.61 ns and τ2 = 940.63 ns, respectively. When the perovskite is deposited on FTO/SnO2, both τ1 and τ2 decrease to 10.90 and 147.50 ns. For the FTO/Nb:SnO2/ perovskite sample, both τ1 and τ2 are further reduced to 6.18 and 45.49 ns, respectively. Compared to the FTO/SnO2/ perovskite sample, the amplitude of the relatively long decay time is reduced to 64.10%, while the corresponding amplitude

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13362. 2426


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The electrical conductivity of the SnO2 and Nb:SnO2 films, the current J−V curves based on SnO2 and Nb:SnO2 films fitting with the Mott−Gurney law, dark I−V curves of the electron-only devices revealing VTFL kink point behavior, top-view SEM image and XRD spectrum of (FAPbI3)0.85(MAPbBr3)0.15 film, the J−V curves of the PSCs based on different thickness of SnO2 ETLs and the Nb:SnO2 ETL with different Nb content, the IQE and stability of the PSCs based on the SnO2 and Nb:SnO2 ETLs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (D.Y.) *E-mail: [email protected]. (S.L.L.) ORCID

Shengzhong Frank Liu: 0000-0002-6338-852X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge all support from the National Natural Science Foundation of China (61604090 and 61674098), the China Postdoctoral Science Foundation funded project (2015M580809), the Changjiang Scholar and Innovative Research Team (IRT_14R33), the National Key Research Program of China (2016YFA0202403) and the Chinese National 1000-talent-plan program.

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